Systems and methods for gradient-modulated pointwise encoding time reduction with radial acquisition magnetic resonance imaging

Abstract

Systems and methods for pointwise encoding time reduction with radial acquisition (“PETRA”) magnetic resonance imaging (“MRI”) using a gradient modulation scheme to enable higher readout bandwidth while keeping the missing samples of the central region of k-space small are provided. This acquisition scheme allows independent selection of the excitation and readout bandwidths, which allows a higher readout bandwidth while keeping the required number of missing central k-space samples low. This flexibility in selecting the excitation and readout bandwidth settings can mitigate the peak radio frequency power and specific absorption rate limitations on flip angle in traditional PETRA imaging schemes.

The invention claimed is: 
     
       1. A method for producing an image with a magnetic resonance imaging (MRI) system, the steps of the method comprising:
 (a) applying a radio frequency (RF) excitation pulse to a subject; 
 (b) acquiring first data from the subject using the MRI system to sample a central region of k-space during a first time period occurring after the RF excitation pulse, wherein a constant magnetic field gradient is applied to the subject during the first time period while the first data are acquired; 
 (c) acquiring second data from the subject using the MRI system to sample an outer region of k-space during a second time period occurring after the RF excitation pulse, wherein an amplitude-modulated magnetic field gradient is applied to the subject during the second time period while the second data are acquired; and 
 (d) reconstructing an image of the subject from the first data and the second data. 
 
     
     
       2. The method as recited in  claim 1 , wherein the first data are acquired by sampling the central region of k-space using a single point imaging (SPI) acquisition. 
     
     
       3. The method as recited in  claim 1 , wherein the second data are acquired by sampling the outer region of k-space using a radial acquisition. 
     
     
       4. The method as recited in  claim 1 , wherein the second data are acquired by sampling the outer region of k-space using a spiral cone acquisition. 
     
     
       5. The method as recited in  claim 1 , wherein the RF excitation pulse is applied with an excitation bandwidth that is different from a readout bandwidth associated with the amplitude-modulated magnetic field gradient applied while the second data are acquired. 
     
     
       6. The method as recited in  claim 5 , wherein the excitation bandwidth is lower than the readout bandwidth. 
     
     
       7. The method as recited in  claim 1 , wherein the first period of time is associated with a delay time having a beginning that coincides with when a receiver gate in the MRI system opens to begin data acquisition. 
     
     
       8. The method as recited in  claim 7 , wherein the delay time begins after a transmit/receive switching time of the MRI system. 
     
     
       9. The method as recited in  claim 1 , wherein applying the amplitude-modulated magnetic field gradient includes increasing a magnetic field gradient from a first value to a second value. 
     
     
       10. The method as recited in  claim 9 , wherein the first value is an amplitude of the constant magnetic field gradient. 
     
     
       11. The method as recited in  claim 10 , wherein the central portion of k-space is defined based on the amplitude of the constant magnetic field gradient and a time from a center of the RF excitation pulse to when a receiver gate of the MRI system opens. 
     
     
       12. The method as recited in  claim 11 , wherein the central portion of k-space is defined as points in k-space less than γG ex ·t k , wherein γ is a gyromagnetic ratio, G ex  is the amplitude of the constant magnetic field gradient, and t k  is the time from a center of the RF excitation pulse to when a receiver gate of the MRI system opens. 
     
     
       13. A magnetic resonance imaging (MRI) system, comprising:
 a magnet for generating a polarizing magnetic field, B 0 , in which a subject to be imaged is positioned; 
 a radio frequency (RF) coil; 
 a gradient coil assembly comprising at least one gradient coil; 
 a processor in communication with the RF coil and the gradient coil assembly; and 
 wherein under instructions received from the processor the RF coil, and the gradient coil assembly operate to perform a pulse sequence that includes:
 (a) applying an RF excitation pulse using the RF coil; 
 (b) acquiring first data using the RF coil to sample a central region of k-space during a first time period occurring after the RF excitation pulse, wherein a constant magnetic field gradient is applied by the gradient coil assembly during the first time period while the first data are acquired; 
 (c) acquiring second data using the RF coil to sample an outer region of k-space during a second time period occurring after the RF excitation pulse, wherein an amplitude-modulated magnetic field gradient is applied by the gradient coil assembly during the second time period while the second data are acquired. 
 
 
     
     
       14. The MRI system as recited in  claim 13 , wherein the processor is configured to operate the RF coil and the gradient coil assembly to acquire the first data using a single point imaging (SPI) acquisition. 
     
     
       15. The MRI system as recited in  claim 13 , wherein the processor is configured to operate the RF coil and the gradient coil assembly to acquire the second data using a radial acquisition. 
     
     
       16. The MRI system as recited in  claim 13 , wherein the processor sets an excitation bandwidth that is different from a readout bandwidth. 
     
     
       17. The MRI system as recited in  claim 16 , wherein the processor sets the excitation bandwidth as lower than the readout bandwidth. 
     
     
       18. The MRI system as recited in  claim 13 , further comprising a receiver gate in communication with the RF coil, and wherein the first period of time is associated with a delay time having a beginning that coincides with when the receiver gate opens to begin data acquisition of the first data. 
     
     
       19. The MRI system as recited in  claim 13 , wherein the processor operates the gradient coil assembly to apply the amplitude-modulated magnetic field gradient by increasing a magnetic field gradient from a first value to a second value. 
     
     
       20. The MRI system as recited in  claim 19 , wherein the first value is an amplitude of the constant magnetic field gradient.